The invention relates to a television picture display device comprising a picture display tube having a scanning circuit which can be synchronized by a first horizontal synchronizing signal and a first vertical synchronizing signal from a first video signal source for obtaining a main picture display of a first video signal obtained from the first video signal source, and a subpicture change-over switch which can be operated by a switching signal for alternately applying the first video signal or a sub-picture signal to the picture display tube, said sub-picture signal being obtained by means of a compression circuit from a second video signal supplied by a second video signal source, the compression circuit comprising a field memory having a write and a read circuit, which write circuit can be synchronized by a second vertical synchronizing signal obtained from the second video signal source and by a second horizontal synchronizing signal divided in frequency by a divider circuit.
A television picture display device of the type described above is known for example from IEEE Transactions on Consumer Electronics, February 1979, pages 512-519.
Since the video signal sources are generally not synchronized, the information of the sub-picture displayed during one and the same field of the main picture mostly originates partly from the actual field and partly from the previous field of the second video signal source. This causes disturbing phenomena such as, for example, fringes on oblique picture elements.